Flexography is a method of printing that is commonly used for high-volume printing runs. It is usually employed for printing on a variety of substances particularly those that are soft and easily deformed, such as paper, paperboard stock, corrugated board, polymeric films, fabrics, plastic films, metal foils, and laminates. Course surfaces and stretchable polymeric films can be economically printed by the means of flexography.
Flexographic printing plates are sometimes known as “relief printing plates” and are provided with raised relief images onto which ink is applied for application to the printing substance. The raised relief images are inked in contrast to the relief “floor” that remains free of ink in the desired printing situations. Such printing plates are generally supplied to the user as a multi-layered article having one or more imagable layers coated on a backing or substrate. Flexographic printing can also be carried out using a flexographic printing cylinder or seamless sleeve having the desired raised relief image.
In order to accommodate the various types of substrates, flexographic printing plates generally have a rubbery or elastomeric nature whose precise properties are adjusted for a particular substrate and printed surface.
Flexographic printing plates have been prepared in a number of ways. Initially, flexographic printing plates were made by cutting a relief image into a sheet of rubber with a knife. An improvement was achieved by forming a mold that could be produced by photo-etched graphics and then by pouring molten rubber into a mold and vulcanizing to form the printing plate. More recently, relief images have been prepared by exposing photosensitive compositions coated on the substrate through a masking element or transparency and then removing non-exposed regions of the coating with a suitable solvent. Various photosensitive compositions are known for this purpose including those containing photosensitive polymers and polymerizable monomers.
U.S. Pat. No. 4,323,636 (Chen) describes the use of thermoplastic elastomeric block copolymers (often sold under the trademark of KRATON®) in combination with photosensitive components in a composition that can be laminated or extruded onto a substrate.
U.S. Pat. No. 5,719,009 (Fan) describes a way to avoid the use of the masking layer to provide a flexographic printing plate. The elements having an ablatable layer disposed over photosensitive layer(s) so that after image ablation, UV exposure of the underlying layer hardens it while non-exposed layer(s) are washed away. DuPont's Cyrel® FAST™ thermal mass transfer plates are commercially available ablatable elements that require no chemical processing, but they do require thermal wicking or wiping to remove the non-exposed areas.
Radiation-sensitive elements having a laser-ablatable mask layer on the surface are known in the art. A relief image can be produced in such elements without the use of a digital negative image or other imaged element or masking device. A masking element is imagewise ablated to form and then placed in contact with a radiation-sensitive element and subjected to overall exposure with actinic radiation (for example, UV radiation). The combined elements are then “developed” to remove the masking element and unexposed regions of the resulting flexographic printing plate. A significant advance in this technique for making flexographic printing plates is described in U.S. Patent Application Publication 2005/0227182 (Ali et al.).
However, there remains a desire in the art to find a way to make flexographic printing plates by direct thermal imaging, thereby avoiding the need for masking elements or devices. Difficulties arise with this approach because most imaging devices have insufficient power to provide sufficient relief depth. Moreover, as the relief depth is increased, a greater volume of volatiles and debris are created that must be contained in an environmentally acceptable manner.
Direct laser engraving is described, for example, in U.S. Pat. Nos. 5,798,202 and 5,804,353 (both Cushner et al.) in which various means are used to reinforce the elastomeric layers. Elastomeric foams are described in similar elements in U.S. Pat. Nos. 6,090,529 and 6,159,659 (Gelbart). Engravable elements containing hydrocarbon-filled plastic and heat-expandable microspheres are described in U.S. Patent Application Publication 2003/0180636 (Kanga et al.).
Commercial laser engraving is typically carried out using carbon dioxide lasers. While they are generally slow and expensive to use and have poor beam resolution, they are used because of the attractions of direct thermal imaging. However, it would be preferable to use infrared (IR) diodes for infrared radiation engraving that have the advantages of high resolution and relatively lower cost so that they can be used in large arrays. Other IR lasers, such as fiber lasers, are also useful. IR laser engravable flexographic printing plate blanks having unique engravable compositions are described in WO 2005/084959 (Figov).
Laser ablatable image transfer elements or masking elements and methods of use include the use of ablatable polymers such as poly(cyanoacrylate), polycarbonates, or polyols in combination with a colorant or pigment that can be transferred. Such elements and methods are described for example, in U.S. Pat. Nos. 5,605,780 (Burberry et al.), 5,998,088 (Robello et al.), 5,712,079 (Robello et al.), 5,156,938 (Foley et al.), and U.S. Patent Application Publication 2003/0020024 (Ferain et al.).
While there have been a number of advances in the art relating to laser-ablatable elements, there remains a need for ablatable compositions and elements that break down “cleanly” during laser imaging (or engraving) to produce fewer but identifiable components and minimal debris, thus providing better control of the imaging process and environmental and health factors. There is particularly a need for laser-ablatable elements that can be imaged in this manner to provide flexographic printing plates with sufficiently deep relief images.